I'm trying to assemble a Power supply for a Nichrome wire heater

Thread Starter

birckcmi

Joined Jan 1, 2018
91
Hi. I'm trying to assemble the components to drive a single-Nichrome-wire device for heating sheets of thermoplastic. To get the material up to 350˚F should take about 3-4 amps with 20 gauge wire, so the power supply needs to deliver at least 5 amps. I know how an AC-to-DC power supply functions (transformer-rectifier-filter capacitors-voltage regulator), but I have no engineering background. The device can be AC or DC. I've heard arguments for both. I'd like to wind up with an adjustable power supply that can be used for other purposes as well, e.g., battery recharging, LED power, etc. I've gone looking for new and used off-the-shelf 5 amp power supplies, but the good used American-made 5A-10A supplies (B&K, Elenco, HP) are thin on the ground, and the New market is flooded with Chinese-built equipment bristling with inflated claims of capability and erroneous/doubtful/incomprehensible User Manuals. 100 different brand names all apparently coming from the same factory.
Sorry. I'm hoping that I can get recommendations for assembling components to power a nichrome heater. Do all of them need to be the same amperage? Should I regulate the incoming AC voltage, or just the DC out? Would I be better off just buying a Variac and sticking with AC?
 

dl324

Joined Mar 30, 2015
10,055
Trying to have a power supply that can deliver 5A with variable voltage won't be cheap. What is the voltage adjustment range you think you need?

A computer power supply will provide 12V at 10's of amps inexpensively. You can add linear regulators to the 12V output for voltages under about 9V. If you need higher voltages, you can use boost regulators.
 

Reloadron

Joined Jan 15, 2015
5,128
A heating element can make heat using AC or DC so there is really no need to rectify the AC, think about a counter top kitchen toaster as an example.

While the use of a Variac is a fine solution and would be my choice the forum rules preclude any discussion where AC mains power is not isolated and using a Variac provides no mains power isolation so I need to pass on that line of thought. That said a Variac driving a step down transformer would be a viable solution. This would afford mains isolation.

What you need to do is get a nichrome wire resistivity chart, Chart of Properties of Nichrome Wire would be a good start. Then you need to calculate the heat for the area you wish to heat. Yes, a good used HP or similar US manufactured supply comes at a cost but with any luck you can find something online. You also need to consider the ampacity limits of AWG 20 Nichrome Wire to derive your voltage and current limits. Also how you plan to insulate the wire from whatever you are heating.

Ron
 

Thread Starter

birckcmi

Joined Jan 1, 2018
91
Trying to have a power supply that can deliver 5A with variable voltage won't be cheap. What is the voltage adjustment range you think you need?

A computer power supply will provide 12V at 10's of amps inexpensively. You can add linear regulators to the 12V output for voltages under about 9V. If you need higher voltages, you can use boost regulators.
dl324, thanks for the excellent question "what is the voltage adjustment range...". It's excellent because I can't answer it. I have some idea how amperage would determine the heat the wire could produce, and I understand that voltage and amperage can "convert", but I don't know how voltage-high or low- affects a heater wire. I do know that a 20-AWG wire would require 3.8 amps to reach 400˚F, and I would like to be able to put out 10 amps at 12 volts for charging a car battery. Can you suggest a voltage range?
 

Thread Starter

birckcmi

Joined Jan 1, 2018
91
A heating element can make heat using AC or DC so there is really no need to rectify the AC, think about a counter top kitchen toaster as an example.

While the use of a Variac is a fine solution and would be my choice the forum rules preclude any discussion where AC mains power is not isolated and using a Variac provides no mains power isolation so I need to pass on that line of thought. That said a Variac driving a step down transformer would be a viable solution. This would afford mains isolation.

What you need to do is get a nichrome wire resistivity chart, Chart of Properties of Nichrome Wire would be a good start. Then you need to calculate the heat for the area you wish to heat. Yes, a good used HP or similar US manufactured supply comes at a cost but with any luck you can find something online. You also need to consider the ampacity limits of AWG 20 Nichrome Wire to derive your voltage and current limits. Also how you plan to insulate the wire from whatever you are heating.

Ron
Reloadron, thank you for the note about isolating the Variac output with a step-down transformer. I hope to avoid self-immolation. I'm working from a current/temperature table provided by one of the Nichrome wire mfr's. It doesn't calculate ohms, though. Is that an important consideration?
 

Thread Starter

birckcmi

Joined Jan 1, 2018
91
For heating sheets I use my kitchen oven, for cutting foam a Variac feeding a 120 to 24V, 100W transformer.
Bernard, thanks for the note. So the foam cutter is powered by AC, to a 24VAC, 100 watt transformer alone, or is "transformer" shorthand for a device like a buck converter or a voltage regulator?
 

Lo_volt

Joined Apr 3, 2014
97
There's no doubt that a variac, as Reloadron suggested, will be the simplest solution. It's been a while since I did it, but I worked with such a heater to fold plastic sheets.

A couple of things to note, make sure the variac has sufficient VA rating to supply the power you'll need and think about interlocking and fusing your setup so that hands cannot be near exposed voltage or the nichrome wire while it's in operation. Also, if you are using large sheets, be aware of drafts and air currents that may cool sections of the sheet. You may need to add baffles to defeat drafts.
 

Thread Starter

birckcmi

Joined Jan 1, 2018
91
Lo_Volt, thanks for the note. The answers are trending toward using a variac. When I looked at how-to's on YouTube, the first one I saw used AC power through a variac, but most of the others were DC circuits, I guess for safety. Regarding safety, most of the how-tos were pretty godawful, with free-hanging, un-insulated powered wires in close proximity to aluminum channel and steel hinges, not to mention the operator. One of the DIY plastic-bender videos actually specified running the wire 1/4" above a large brass sheet covering the plywood base. Why the brass sheet? To act as a heat-shield for the plywood!
 

Thread Starter

birckcmi

Joined Jan 1, 2018
91
I'm a rank beginner at this. I'm trying to assemble a heating circuit for a plastic bender. The nichrome wire supposedly requires 3.8 Amps to reach 400˚F, but is that AC or DC? IF AC, that combination results in 486 watts. If DC, the same combination (assuming 12 volts DC) yields 48 watts. How can this be right? Can somebody set me straight?
 

dl324

Joined Mar 30, 2015
10,055
\[ \]
is that AC or DC? IF AC, that combination results in 486 watts. If DC, the same combination (assuming 12 volts DC) yields 48 watts. How can this be right? Can somebody set me straight?
It can be AC or DC. If you use AC, you wouldn't use 120VAC.

\[ P = IV = \frac{V^2}{R} => R = \frac{V^2}{IV} = \frac{V}{I} = \frac{12V}{3.8A} = 3.16 ohms \]
\[ P = \frac{V^2}{R} => V = \sqrt{PR} = \sqrt{45.6W*3.16ohms} = 12V \]
So you'd need a 12VAC source.

EDIT: I realized after jumping through hoops to get Latex to work that the calculations weren't required because power dissipation from 12VDC and 12VAC would be equivalent...:mad:
 
Last edited:

Bernard

Joined Aug 7, 2008
5,397
How long is the wire ? Variacs are expensive. A microwave transformer stripped of all windings
except primary can be rewound with taps. I think it provides about 2 to 3 turns / V. Taps can be brought out to Cu segments or brass bolt heads in a circular pattern. A rotatable arm then selects
V.
 

wayneh

Joined Sep 9, 2010
16,254
Safety First! Don't actually do anything until you understand this and know the risks. Hooking your wire to 120V AC will probably light it up like a flashbulb, for a split second, followed by a puff of smoke.

You need a power supply that can limit current flow. The voltage required for a particular current, say 3A, is unknown from what you've provided so far. The length and gauge of the wire needs to be known, so that the resistance can be estimated. For instance suppose it's expected to have a resistance of 2Ω at 400°F. Using Ohm's law V = I•R (for units of volts, amps and ohms). So V = 3A • 2Ω = 6 volts. You could go then buy a power supply with variable current, max voltage 9V or 12V. If the resistance is 20Ω, V = 3A•20Ω = 60 volts. That means your power supply would need to be capable of 3A at 60V, or 180W.
 

MrChips

Joined Oct 2, 2009
20,263
How do you know that the temperature of the wire will be 400°F?

What is the length of the wire?
What is the wire gauge?
What is the ambient temperature?
How is the wire mounted?
How is heat removed from the wire? Conduction, convection, radiation?
 

Thread Starter

birckcmi

Joined Jan 1, 2018
91
Safety First! Don't actually do anything until you understand this and know the risks. Hooking your wire to 120V AC will probably light it up like a flashbulb, for a split second, followed by a puff of smoke.

You need a power supply that can limit current flow. The voltage required for a particular current, say 3A, is unknown from what you've provided so far. The length and gauge of the wire needs to be known, so that the resistance can be estimated. For instance suppose it's expected to have a resistance of 2Ω at 400°F. Using Ohm's law V = I•R (for units of volts, amps and ohms). So V = 3A • 2Ω = 6 volts. You could go then buy a power supply with variable current, max voltage 9V or 12V. If the resistance is 20Ω, V = 3A•20Ω = 60 volts. That means your power supply would need to be capable of 3A at 60V, or 180W.
 
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